Hydrologic properties of soil and subsoil on a steep, forested slope

Abstract:

The purpose of this study was to examine certain hydrologic
properties of the soil and subsoil on a steep forested slope and relate
these -rceTties to the movement of water via subsurface routes.
The hydrologic properties examined were bulk density, soil texture,
total porosity, pore size distribution, saturated hydraulic conductivity, -
and soil moisture-tension relationships.
Soil samples were taken from a 2. 5 ha study slope on watershed
10 of the H. S. Andrews Experimental Forest near Blue River,
Oregon. Eleven soil pits were excavated and six soil core samples
were taken at depths of 10, 30, 70, 110, 130, 150, and 200 cm where
soil conditions permitted. Laboratory analyses were conducted to
determine the hydrologic properties of each sample. The extreme
permeability and high porosities of the samples necessitated the use
of specially designed apparatus to measure the saturated weights and
hydraulic conductivities.
Particle size distribution changed only slightly with depth. The
A and B horizons were predominately clay barns and the C horizons
were classified as clays. Total porosities also varied little with depth.
The porosity of the soil (A and B horizons) averaged nearly 65% while
the porosity of the subsoil (C horizons) averaged nearly 55%. Bulk
density also varied little with depth. Soil bulk densities averaged
825 gm/cm3 and subsoil bulk densities averaged 1. 180 gm/cm3.
The hydraulic conductivity and pore size distribution of the soil
and subsoil were well. correlated and changed considerably with depth.
Significant decreases in the hydraulic conductivities occurred between
the 30 cm and 70 cm depths as well as between the 110 cm and 130 cm
depths in some of the soil pits. At most soil pits the surface soil had
conductivities greater than 400 cm/hr while the soil at the 70 cm and
110 cm depths had conductivities near 200 cm/hr. Subsoils had much
lower conductivities, less than 60 cm/hr in most soil pits and less
than 10 cm/hr in some pits. A power curve regression analysis was
'S. used to relate the hydraulic conductivity (Y) and the mean percentage
of pores greater than . 294 mm in diameter (X) according to the equa-
Y 10, 040X2997 . The resulting r2 was .945. The percentage of
pores greater than . 294 mm in diameter was also found to change
abruptly between the 30 cm and 70 cm depths in most soil pits and
between the 110 cm and 130 cm depths in some soil pits.
The hydrologic properties were used to discuss the possible
nature of water movement through the soil and subsoil of the study
slope. The soil hydrologic properties and antecedent moisture conditions
were predicted to be conducive to vertical unsaturated translatory
flow. A zone of saturation was predicted to occur during winter
rainfall events above the subsoil horizon having extremely low conductivity
rates (above the 130 cm depth near soil pit 1). This zone of
saturation was predicted to be the most probable zone of lateral water
movement in the form of saturated translatory flow.
Data from a soil pit known to have saturated flow over the subsoil
and from tensiometers installed near the soil pits were presented
as evidence that a zone of saturation does exist within the subsoil
during some rainfall events and that the soil and subsoil moisture
conditions are conducive to translatory flow during the winter rainy
season.